1. Field of the Invention
The present disclosure relates to a piezoelectric thin film element for use in an inkjet recording head or the like, and a method of manufacturing the piezoelectric thin film element, and to a droplet discharge head and an inkjet recording device using the piezoelectric thin film element.
2. Description of the Related Art
Some of conventional image recording devices and imaging devices such as a printer, a facsimile, and a copy machine use an inkjet recording device equipped with an inkjet recording head as a liquid discharge head (liquid ejection head). Two types of inkjet recording heads (ink ejection heads) are practically used: one uses a longitudinal vibration mode piezoelectric actuator which extends and contracts in the axial direction of a piezoelectric film (hereinafter referred to as a piezoelectric material or a piezoelectric thin film as needed), and the other uses a flexural vibration mode piezoelectric actuator.
An inkjet recording head using the longitudinal vibration mode piezoelectric actuator has a piezoelectric element (piezoelectric thin film element) in which an electromechanical conversion element is stacked on a vibration plate. The electromechanical conversion element has a longitudinal vibration mode piezoelectric film (piezoelectric material) extending and contracting (expanding and contracting) in the axial direction (longitudinal direction), and electrodes between which the piezoelectric film (piezoelectric material) are held in a vertical direction. The inkjet recording head is configured to eject droplets of an ink in a compression cavity through nozzles by applying pressure by expansion and contraction of the piezoelectric film to the ink in the compression cavity.
On the other hand, in a known inkjet recording head using the flexural vibration mode actuator, for example, piezoelectric elements are formed independently in respective pressure generation cavities in such a way that a uniform piezoelectric film is formed as a piezoelectric material layer over the entire surface of a vibration plate by film formation technology, and the piezoelectric material layer is cut into shapes corresponding to the pressure generation cavities by the lithography technique.
Japanese Patent Application Publication No. 2004-186646 discloses a technique of enabling formation of a piezoelectric film having superior crystal orientation by forming a piezoelectric film on a titanium-containing noble metal electrode on the surface of which titanium is deposited in island shapes. Japanese Patent Application Publication No. 2004-262253 discloses a technique of enabling formation of a piezoelectric film having superior crystal orientation by using a MgO substrate as a substrate. Furthermore, Japanese Patent Application Publication No. 2003-218325 discloses a method of manufacturing a ferroelectric film as a piezoelectric film, by forming an amorphous ferroelectric film, and thereafter crystallizing the film by rapid heating method, Japanese Patent Application Publication No. 2007-258389 discloses a method of manufacturing a piezoelectric film which includes a perovskite-type complex oxide (may also contain unavoidable impurities) having a crystal structure of one of tetragonal system, orthorhombic system, and rhombohedral system, and is has a preferred orientation with any one of (100) plane, (001) plane, and (111) plane at a degree of orientation of 95% or higher in the above film formation process.
In most of the piezoelectric films in the above-mentioned Japanese Patent Application Publications, a PZT film is produced on platinum. A metal material such as Pt, Ir, Ru, Ti, Ta, Rh, or Pd has been used as an electrode material conventionally. Among them, platinum is generally used. The reason for the frequent use of platinum is that platinum has a high self-orientation due to its face-centered cubic lattice (FCC) structure which is a close-packing structure; and even when a platinum film is formed on an amorphous material such as SiO2 which is a material of the vibration plate, the platinum film is highly oriented with the (111), and the piezoelectric film on the platinum film also has a favorable orientation.
However, columnar crystal grains grow due to a high orientation, and thus there is a problem in that an element such as Pb is likely to diffuse to the base electrode along grain boundaries.
In addition, in the conventional technology, a possibility has been pointed out that oxygen deficiency in the piezoelectric thin film (piezoelectric film, piezoelectric material) may increase over time during the operation of the piezoelectric thin film (piezoelectric film, piezoelectric material). A conductive oxide electrode is used today as the supply source for the deficient oxygen.
In order to form the piezoelectric thin film, a conductive oxide electrode immediately below the piezoelectric thin film plays a key role. Among all, SrRuO3 (strontium ruthenate) has the same perovskite-type crystal structure as PZT. Thus, SrRuO3 has good bondability at the interface, allows easy epitaxial growth of PZT, and provides excellent characteristics of Pb diffusion barrier layer (for example, see Japanese Patent No. 3249496, Japanese Patent No. 3472087, and Japanese Patent Application Publication No. Hei 11-195768).
A lower electrode provided on the above-described vibration plate has been formed by using a stacked structure in which titanium, platinum, and conductive oxide SrRuO3 films are formed in this order on the vibration plate. The reason why the titanium film is used is to improve adhesion between the vibration plate and the lower electrode. Observation with a scanning electron microscope has revealed that when the titanium film is used as an adhesion layer, minute holes each having a dimension of 100 nm or less are generated in the lower electrode film and on the surface of the lower electrode after the formation of the SrRuO3 film. This is because titanium diffuses into the platinum film when the SrRuO3 film is formed at a high temperature, and thus holes are formed in the platinum film by the diffusion.
Such a SrRuO3 film is generally formed by sputtering method. The formation of a SrRuO3 film having a favorable crystalline state and highly oriented with the (111) plane by the sputtering method requires a film formation at a high temperate of 450 to 650° C. Thus, after the temperature is increased to 450 to 650° C., the SrRuO3 film is formed by sputtering method.
However, with the sputtering method, when the temperature is increased to 450 to 650° C. before the formation of the SrRuO3 film, titanium diffuses into the platinum film, and holes are formed in the platinum film as described above.
When holes are formed in the platinum film in this manner, the continuity of a conductive oxide crystal is blocked due to the holes, and thus it is difficult to form the conductive oxide SrRuO3 film having an average particle diameter greater than that of the platinum film. In this case, the crystalline state of the conductive oxide crystal affects the quality of the piezoelectric thin film (piezoelectric material) which is formed immediately above the conductive oxide crystal.
Therefore, what are necessary to further improve the characteristics of the piezoelectric element (piezoelectric thin film element) having the piezoelectric thin film are to increase the size of the crystal grains of the conductive oxide SrRuO3 film and to achieve favorable crystalline state.
On the other hand, when the conductive oxide SrRuO3 film has a small average particle diameter, and has a high surface roughness of the conductive oxide, the conductive oxide SrRuO3 has more grain boundaries. As the number of grain boundaries of the conductive oxide SrRuO3 increases, a leak current increases. In addition, the holes in a conductive oxide SrRuO3 with a high surface roughness cause electric field concentration, thereby reducing the breakdown voltage of the piezoelectric element in which a piezoelectric thin film is interposed between a pair of electrodes.
In addition, since holes are generated in the platinum film, a lead component in the PZT film is excessively trapped in the holes in the surface of the lower electrode and in the holes in the platinum. Therefore, in the vicinity of the lower electrode, the lead component in sol gel liquid needs to exceed the required amount according to stoichiometry by 20 mol %.
In the above piezoelectric element, the excessive diffusion of Pb into the lower electrode causes the formation of a leak path and the concentration of electric field, thereby reducing the breakdown voltage of the piezoelectric element. In addition, an inkjet recording head (ink ejection head) using the piezoelectric element as the piezoelectric actuator causes reduction in the speed of droplet.
In order to eliminate the above-described problems, the holes in the surface of the lower electrode may be removed by increasing the thickness of the platinum film. However, an increase in the thickness of the platinum electrode made of an expensive metal causes an increase in cost, and thus is not desirable.
Instead, in order to eliminate the above-described problems, the amount of titanium that diffuses into the platinum film may be reduced by decreasing the thickness of the titanium film. However, extremely reduced thickness of the titanium film leads to decrease in the adhesion of the titanium film, and may cause the lower electrode to come off.